Transmitter and/or receiver (henceforth referred to generically as "transceiver") technology has evolved over the decades from the use of wires, electro-mechanical components, and machined waveguide structures to the use of coax and thick film/thin film microstrip/stripline-based circuitry. But even with this evolution, the recent proliferation of, and resulting stiff competition among, wireless communications products have led to price/performance demands on transceivers that conventional technologies find difficult to meet. And some of the more expensive components of a transceiver are the "front end" filters.
Planar filters have been of interest to transceiver designers in recent years because of their relatively small size, low cost, and ease of manufacture. A planar filter is generally implemented using flat transmission-line structures, such as microstrip and stripline transmission lines separated from a ground plane by a dielectric layer. A typical implementation defines the planar filter as conductive traces on one side of a printed circuit (PC) board, defines the ground plane as a conductive layer on the other side of the PC board, and uses the laminate of the PC board for the dielectric. An illustrative example of such a planar filter is disclosed in U.S. Pat. No. 5,990,765.
Although the use of planar filters is advantageous, the planar-filter designs known to the inventors do not take sufficient advantage of the filter configuration and layout to maximize filter performance.